Pulse width modulation power converter and control method employing different sets of PID coefficients

ABSTRACT

In a Pulse Width Modulation power converter and control method, wherein one of the operating modes steady state or load transient is detected. For either of the two operating modes one set of PID coefficients is provided for the control law that controls the duty ratio command. In case a load transient is detected, the KP gain is selected adaptively. Operating mode detection is supported by oversampling the error signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under section 371 ofInternational Application No. PCT/EP2011/069725, filed on Nov. 9, 2011,and published in English on May 18, 2012, as WO 2012/062795 and claimspriority of European application No. EP 10190571.9 filed on Nov. 9,2010, the entire disclosure of these applications being herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an apparatus and a method forgenerating pulse width modulation (PWM) signals. The present inventionspecifically relates to an apparatus and a method for generating PWMsignals in switching regulators or power converters.

BACKGROUND ART

A switching regulator is a circuit that uses a power switch, aninductor, and a diode to transfer energy from input to output. Incontrast to linear regulators that use a resistive voltage drop toregulate the voltage and lose power in the form of heat a switchingregulator does not have a voltage drop and an associated current butinstead the current is 90 degree out of phase with the voltage. Due tothis, the energy is stored and can be recovered in the discharge phaseof the switching cycle. In the art, several types of switchingregulators exist dependent on how the switching circuit is arranged, forexample step-down (buck), step-up (boost) or inverter (flyback). Theswitch can be controlled by a PWM signal with duty ratio D thatrepresents the on state during a PWM period. The output voltage isdependent on the duty ratio D and, hence, can be controlled by acontroller which consists of an analog-to-digital-converter (ADC), adiscrete-time control law, and a digital PWM (DPWM) module. The ADCsamples and quantizes the regulated signal, i.e., the output voltageerror that is the difference between the output voltage and an outputvoltage reference. The control law computes the digital duty ratiocommand D based on the quantized output voltage error. The control lawis a given by a PID control law which is configured by a set of PIDcoefficients, the set comprising the proportional gain Kp, the integralgain Ki and the differential gain Kd and the time delay Td. The digitalPWM modulator takes D as input, and outputs a PWM waveform with thecommanded duty ratio D at a switching frequency. The PWM waveform hasfinite time resolution. The sensing and the quantization of othersignals such as the load can be added depending on the application andthe specific control law used.

In the state of the art, one set of PID coefficients is used. In case alarge load transient occurs the response of the PID controller may beslow.

DISCLOSURE OF THE INVENTION

It is therefore an objective of the present invention to provide anapparatus and a method for generating PWM signals that lead to asuperior response to load transients over the prior art.

One aspect relates to a pulse width modulation power converter,comprising an output stage that generates an output voltage according toa pulse width modulation signal and an input voltage by means of aswitching element. The PWM modulator further comprises means forproviding an output voltage reference and an analog digital converterconnected to the output stage and the means for providing an outputvoltage reference. The ADC samples and amplifies a difference betweenthe output voltage and the output voltage reference to generate an errorsignal. A PID controller connected to the ADC and the switching elementwherein the PID controller is configured by a set of PID coefficientsdetermines a duty ratio for a pulse width modulator that generates thePWM signal. The PWM power converter further comprises means fordetecting a steady state and a load transient. Two different sets of PIDcoefficients will be provided for the PID controller, a first set forthe steady state and a second set in case a load transient is detected.The two sets are stored in a memory and one of them is selected byselector means.

One aspect of the present invention relates to means for detecting asteady state or a load transient. These means for detecting a steadystate or a load transient comprise means for monitoring the error signaland comparing each value of the error signal to its predecessor bygenerating an error signal difference, preferably by taking into theaccount the second derivative of the error signal.

One aspect of the present invention relates to the PWM modulator and itstriggering. A duty ratio difference of the duty ratio and itspredecessor is computed and, in case during an off-time of the pulsewidth modulation signal, the duty ratio difference exceeds a threshold,the pulse width modulator is triggered to start a new pulse widthmodulation period. Hence, the response of the modulator to an outputvoltage drop is superior in terms of speed since the modulator isprovided with a refreshed duty ratio before the current PWM period ends.In case the load transient is detected during the on-time of the PWMperiod the on-time may be simply prolonged.

One further aspect of the present invention relates to the second set ofPID coefficients for the transient mode. For the proportional gain Kp, aplurality of nonlinear gain coefficients are provided in a storagemeans. A nonlinear gain coefficient is adaptively selected dependent onthe load transient.

One aspect of the present invention relates to oversampling. The pulsewidth modulation power converter comprises means for providing theanalog-to-digital converter with a first clock frequency and the pulsewidth modulator with a second clock frequency, wherein the first clockfrequency is higher than the second clock frequency.

One further aspect of the present invention relates to oversampling. ThePWM power converter comprises a moving average filter for computing amoving average of the error signal.

The present invention further relates to a control method for a powerconverter wherein an output voltage is generated according to a pulsewidth modulation signal and an input voltage. An error signal isgenerated by sampling the output voltage and differencing the sampledoutput voltage and an output voltage reference. A duty ratio thatdefines a duty cycle of pulse width modulation signal is determined bymeans of PID controlling algorithm generating a pulse width modulationsignal by providing the duty ratio to a pulse width modulator.

A steady state or a load transient is detected. A first set of PIDcoefficients is selected in case a steady state is detected and a secondset of PID coefficients is selected in case a load transient isdetected.

The step of detecting a steady state or a load transient may comprisemonitoring the error signal and comparing each value of the error signalto its predecessor by computing an error signal difference, preferablytaking into account the second derivative of the error signal. If theerror signal difference exceeds a threshold a load transient isdetected.

A duty ratio difference of the duty ratio and its predecessor may becomputed and, in case during an off-time of the pulse width modulationsignal the duty ratio difference exceeds a threshold, the pulse widthmodulator may be triggered to start a new pulse width modulation period.

A nonlinear gain KP may be selected in case of load transient detection.Adapted nonlinear gains KP have the advantage that a steady state isreached more quickly compared to linear gains.

The output voltage signal and/or the error signal may be oversampled bysampling a plurality of error signals within one PWM period. Theoversampling is necessary for the load transient detection since aplurality of duty ratios are compared to its predecessors respectivelywithin one PWM period.

Further, a moving average of the sampled error signal is computed by amoving average filter. This leads to an interpolated error signal withsupport in between samples provided by the ADC. Moreover, by applying amoving average filter the ripple of the error signal is suppressed.

One further advantage of the present invention is that due to a superiorresponse to load transients the size of the capacitors on a printedcircuit board may be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus and method according to the invention are described inmore detail herein below by way of exemplary embodiments and withreference to the attached drawings, in which:

FIG. 1 shows a block diagram of PWM buck converter;

FIG. 2 shows a block diagram of a PWM signal generator;

FIG. 3 shows a block diagram for non-linear KP selection

EMBODIMENT OF THE INVENTION

One embodiment of the present invention is a buck converter as shown inFIG. 1. The buck converter consists of a digital controller 11 and apower train 12. The digital controller comprises an ADC 13, the controllaw 14 and the PWM modulator 15. An error signal 16 is computed bydifferencing the output reference voltage V_ref and the output voltageV_out. The control law takes the error signal 16 as input and outputsthe duty command ratio 17. The duty command ratio 17 controls the PWMmodulator which generates a switching frequency 18. Switching frequency18 and input voltage V_in drive the power train which outputs the outputvoltage V_out.

FIG. 2 shows the digital controller 11 with a circuit 20 for computingand amplifying the error signal, the ADC 13 that quantizes the errorsignal, the control law implemented as PID compensator 14, the PWMmodulator 15 and an additional moving average filter 21 foroversampling. Oversampling is used for detecting load transients andripple suppression. The PID compensator 14 further comprises a unit 26for non-linear KP selection. The ADC clock 22 operates at 16 Mhz, thePID update clock 23 at 48 Mhz and the PWM clock 24 at 500 kHz.

For the PID compensator 14 two sets of PID coefficients are provided,one for the transient mode and one for the steady state mode. Thetransient mode is entered by hardware upon load step transientdetection. In transient mode the PID uses the transient coefficientsKP_T, KI_T, KD_T, TD_T. Furthermore KP is adaptively selected intransient mode. The steady state mode is entered by hardware upon steadystate detection. In steady state the PID uses the steady statecoefficients KP_S, KI_S, KD_S, TD_(—S.)

FIG. 3 shows the unit for non-linear KP selection 26. The unit 26comprises a storage unit 31 for storing the non-linear KP gains and adigital compare and select unit 32 which compares the error signal witha plurality of thresholds that define error ranges, Error Range 0 . . .Error Range 4. For every Error Range 0 to 3 a non-linear KP gain KP_T,KPNL_(—)0.NLC, KPNL_(—)1.NLC, KPNL_(—)2.NLC, KPNL_(—)3.NLC is stored inunit 31. One of these KP gains is chosen by a multiplexer 34 which iscontrolled by the compare and select unit 32. A mode selectionmultiplexer 35 which is connected to the output of the multiplexer 34selects one of the operating modes, steady state or load transient. Themode selection multiplexer 35 is controlled by the mode select signal.The mode select signal is generated by a mode detection unit.

While the present invention has been illustrated and described in detailin the drawings and foregoing description, such illustration anddescription are to be considered illustrative or exemplary and notrestrictive. It will be understood that changes and modifications may bemade by those of ordinary skill within the scope and spirit of thefollowing claims

The invention claimed is:
 1. A pulse width modulation power converter,comprising: an output stage generating an output voltage according to apulse width modulation signal and an input voltage by a switchingelement, a source of an output voltage reference, an analog digitalconverter connected to the output stage and to the source of the outputvoltage reference, sampling and amplifying a difference between theoutput voltage and the output voltage reference to generate an errorsignal, a PID controller connected to the analog digital converter andto the switching element wherein the PID controller is configured by aset of PID coefficients, for determining a duty ratio, a pulse widthmodulator that generates the pulse width modulation signal according tothe duty ratio, a detector for detecting a steady state and a loadtransient, wherein the pulse width modulation converter furthercomprises: a memory for storing a first set of PID coefficients and asecond set of PID coefficients, a mode selector for selecting one of thefirst set of PID coefficients or the second set of PID coefficients forconfiguring the PID controller, the first set of PID coefficients beingselected in case the steady state is detected, and the second set of PIDcoefficients being selected in case the load transient is detected,wherein the detector for detecting the steady state and the loadtransient monitors the error signal and compares each value of the errorsignal to its predecessor by generating an error signal difference; andwherein a duty ratio difference of the duty ratio and its predecessor iscomputed, and, in case during an off-time of the pulse width modulationsignal, the duty ratio difference exceeds a threshold, the pulse widthmodulator is triggered to start a new pulse width modulation period. 2.The pulse width modulation power converter according to claim 1, whereinthe comparing of each value of the error signal to its predecessor bygenerating the error signal difference comprises taking into account asecond derivative of the error signal.
 3. The pulse width modulationpower converter according to claim 1, further comprising a movingaverage filter for computing a moving average of the error signal. 4.The pulse width modulation power converter according to claim 1, furthercomprising a storage unit for storing a plurality of nonlinear gaincoefficients KP of the second set of PID coefficients and adaptivelyselecting a nonlinear gain Kp in case of load transient detection. 5.The pulse width modulation power converter according to claim 1, whereinthe analog digital converter is provided with a first clock frequency,the pulse width modulator is provided with a second clock frequency, andthe first clock frequency is higher than the second clock frequency. 6.A control method for a power converter, comprising: generating an outputvoltage according to a pulse width modulation signal and an inputvoltage; generating an error signal by sampling the output voltage anddifferencing the sampled output voltage and an output voltage reference;determining a duty ratio that defines a duty cycle of the pulse widthmodulation signal by a PID controlling algorithm; generating the pulsewidth modulation signal by providing the duty ratio to a pulse widthmodulator; detecting a steady state or a load transient, selecting afirst set of PID coefficients in case the steady state is detected and asecond set of PID coefficients in case the load transient is detected,and wherein the step of detecting the steady state or the load transientcomprises monitoring the error signal and comparing each value of theerror signal to its predecessor by computing an error signal difference,and computing a duty ratio difference of the duty ratio and itspredecessor, and, in case, during an off-time of the pulse widthmodulation signal, the duty ratio difference exceeds a threshold, thepulse width modulator is triggered to start a new pulse width modulationperiod.
 7. The control method according to claim 6, wherein the step ofcomparing each value of the error signal to its predecessor by computingan error signal difference comprises taking into account a secondderivative of the error signal.
 8. The control method according to claim6, wherein the output voltage and/or the error signal is oversampled bysampling a plurality of error signals within one PWM period.
 9. Thecontrol method according to claim 6, wherein a moving average of thesampled error signal is computed.
 10. The control method according toclaim 6, wherein a nonlinear gain KP is selected in case of loadtransient detection.